Abstract: In recent years, many landslide dams are triggered by frequent tectonic activities and extreme climate. They are seriously threating the lives and properties in both upstream and downstream areas. Constructing spillway is the most common risk mitigation measure. The excavation volume of spillways is largely restricted by the limited available time and extremely terrible transportation conditions. It is urgently demanded to optimize spillway design and reduce the risk of dam failure to the minimum. This paper provides a breaching analysis method for landslide dams by considering the effect of different spillway design, which is applied to a case study of the Tangjiashan Landslide Dam. The instantaneous erosion rate during dam breaching can be obtained based on the interaction of hydraulic and soil erosive parameters. The influence of different spillways on the whole breaching process as well as the optimal spillway design are subsequently achieved. The case study shows that peak outflow rate could be reduced from the recorded 6500m³·s^-1 to 1700m³·s^-1 with the optimal design. The main reason is that a large longitudinal gradient incurs significant erosion in the spillway before dam breach, resulting in two peak outflow rates of largely reduced values. Comparing to a single trapezoidal spillway, combined spillways have smaller hydraulic radius, which reduces the less erosion and higher water level before dam breach, leading to higher peak outflow rate. Therefore, the risk mitigation effect of combined spillways is not as good as that of a single spillway with the same excavation volume.